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Caffeine Effects on Cardiovascular and Neuroendocrine Responses to Acute Psychosocial Stress and Their Relationship to Level of Habitual Caffeine Consumption JAMES D. LANE, PH.D., R. ALISON ADCOCK, B.A., REDFORD B. WILLIAMS, M.D., AND CYNTHIA M. KUHN, PH.D. The effects of a moderate dose of caffeine on cardiovascular and neuroendocrine stress reactivity were examined in 25 healthy male subjects selected as habitual or light consumers of caffeine. Measurements were taken under resting conditions before and after administration of caffeine (3 5 mg/kg) or placebo, during a stressful laboratory task, and in a post-stress recovery period. Caffeine elevated blood pressure and plasma norepinephrine levels at rest, effects which added significantly to the effects of stress. Caffeine potentiated stress-related increases in plasma epinephrine and cortisol stress, more than doubling the responses observed in the control condition. These effects were present in both habitual and light consumers and level of habitual caffeine consumption did not affect their magnitude. Results indicate that caffeine can potentiate both cardiovascular and neuroendocrine stress reactivity and that the habitual use of caffeine is not necessarily associated with the development of tolerance to these effects.

INTRODUCTION

Caffeine is one of the most widely consumed drugs in the world and is certainly the most widely consumed drug in North America and Europe. One survey conducted in the United States in 1976 suggested that 80% of individuals over age 20 years drank coffee on a regular basis, averaging three cups per day (1). Despite such widespread use, caffeine is known to be a stimulant drug that has significant effects on a variety of physiological func-

From the Department of Psychiatry (J.D L., R.A.A., R.B.W.) and the Department of Pharmacology (C.M.K.), Duke University Medical Center, Durham. North Carolina. Address reprint requests to James D. Lane, Ph.D., Department of Psychiatry, Box 3830. Duke University Medical Center, Durham. NC 27710. Received June 19,1989; revision received December 14, 1989.

320 0033-31 74/9O/52O3-O32O$O2 00/0 Copyright © 1990 by the American Psychosomatic Society

tions including central nervous system, cardiovascular, and neurohumoral activity (2). Research conducted over the last several years suggests that, in addition to its many other stimulant effects, caffeine can potentiate or magnify the physiological responses elicited by psychological stressors in the laboratory. These studies suggest that the dietary consumption of caffeine might intensify physiological responses to stresses in everyday life and potentially increase the pathogenic consequences that have been attributed to exaggerated stress reactivity. Caffeine's influence on the pathogenic effects associated with stress reactivity was first observed in mice that lived in a competitive social environment (3). The addition of caffeine to drinking water for several months, or the substitution of brewed coffee or tea, led to increases in plasma renin activity, corticosterone, adrenal weight, and blood pressure. These Psychosomatic Medicine 52:320-336 (1990)

CAFFEINE AND STRESS REACTIVITY

pathophysiologic changes led to significant increases in morbidity and mortality in the mice receiving chronic caffeine compared to the mice that drank water. Caffeine, coffee, or tea had similar effects even when consumed by mice in less stressful environments, except that habitual caffeine consumption increased corticosterone levels only in the high stress environment, evidence of a possible synergistic interaction between dietary caffeine and the level of psychosocial stress. Human studies of caffeine's effects on stress reactivity have focused on cardiovascular reactivity elicited by acute laboratory stressors (4-9]. Caffeine has had consistent effects on blood pressure (BP) in these placebo-controlled studies of healthy, normal subjects, elevating resting BP levels and adding significantly (510 mmHg] to the level of BP reached during exposure to the stressor. As a general rule, the studies suggest that caffeine raises BP during stress by elevating the resting baseline from which the response is measured and not by potentiating the acute BP stress response. Potentiation of cardiovascular reactivity is apparent in other variables however. Caffeine potentiated stress-related increases in cardiac output in one study (9) and forearm blood flow (5, 6) and forearm vasodilation (6) in others. Taken together, the studies suggest that caffeine can indeed influence cardiovascular stress reactivity, either by adding to the level reached during stress or by potentiating the stress response itself. The research to date raises two issues in need of further study. First, the effects of caffeine on neuroendocrine stress responses have received very little attention, despite evidence that caffeine can elevate resting levels of catecholamines and plasma renin activity in humans (10). Psychosomatic Medicine 52:320-336 (1990)

One recent study (9) reported that caffeine potentiated cortisol, but not norepinephrine, responses to a behavioral stressor. Further study of caffeine's effects on neuroendocrine reactivity, including measures of epinephrine reactivity, is certainly warranted, given the importance of catecholamine and cortisol reactivity in the patterns of physiological stress responses mediated by the sympathoadrenal medullary and hypothalamic pituitary adrenal cortical systems. Examination of caffeine effects on neuroendocrine reactivity could also provide evidence pointing to the mechanisms responsible for the effects on cardiovascular reactivity. The second issue arises from evidence that the habitual consumption of caffeine leads to the development of tolerance to the drug's cardiovascular and neuroendocrine effects. The most frequently cited studies (11, 12) do offer evidence that the effects of an acute caffeine dose on resting levels of blood pressure, catecholamines, and plasma renin activity may disappear after several days of chronic administration of high caffeine doses (750 mg/day). However, other studies have shown significant caffeine effects on stress reactivity in habitual caffeine consumers (6-8) as well as in individuals described as "caffeine-naive" (4, 5). Effects on BP and muscle blood flow have been comparable in these two groups, although direct comparisons have not been made. The effects of habitual caffeine use on caffeine/stress interactions deserve further investigation. The extent of any potential pathogenic consequences of caffeine/stress interactions should be directly related to caffeine and stress exposure, but if daily consumption of the drug leads to tolerance to its effects, the negative effects on health attributable to caffeine should be minimal. 321

J. D. LANE ET AL.

The present study had two objectives related to these current issues. To examine caffeine's effects on neuroendocrine stress reactivity, we included measures of plasma catecholamine and cortisol levels in a replication of earlier placebo-controlled studies of caffeine and stress effects on cardiovascular reactivity. To determine whether habitual consumption of caffeine is associated with tolerance to these effects, we compared a group of healthy young men who consumed moderate amounts of caffeine every day with a group of men who consumed very limited amounts of caffeine.

METHODS

sional consumption of tea, iced tea, or caffeinated soft drinks.1 Daily caffeine intake was evaluated first by a questionnaire during the initial visit to the laboratory. Subjects estimated daily consumption of coffee, tea, or caffeinated soft drinks in ounces. These values were converted to mg/day of caffeine using standard estimated values for caffeine content of beverages (14) and were examined to confirm that the subject fit in the category of very light consumption or habitual moderate consumption as described above. Subjects also completed a 1-week diary of caffeine consumption subsequent to laboratory participation They recorded each caffeine-containing product consumed, noting type, quantity, and time of day on a specially printed diary card. Subjects made contemporaneous records when possible, although completion of the day's record in the evening was permitted. Caffeine diary cards were returned by mail at the end of the week. Four subjects did not return diary cards and could not be reached afterwards to provide this data.

Subjects Twenty-five healthy male volunteers (aged 18-36 years) were recruited for participation in the study after telephone screening of respondents to advertisements placed on campus. All subjects had a negative history of cardiovascular disease or other chronic disease that could affect participation. None were under the care of a physician or taking any medications for significant acute ailments at the time of participation. Tobacco smokers were excluded to avoid potential confounding effects of nicotine or nicotine withdrawal. Subjects received $50 for participation in the three sessions of the study.

Assessment of Caffeine Intake Subjects were included in one of two caffeine-use groups on the basis of self-reports of daily caffeine consumption. Twelve subjects were selected as "light caffeine consumers" who reported during telephone screening that they never drank coffee or tea and drank less than one other caffeinated beverage (e.g., a carbonated soft drink) per day. Thirteen subjects wore selected as "habitual moderate caffeine consumers" who reported drinking between three to five cups of coffee every day, along with occa-

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Cardiovascular and Respiratory Measurements Cardiovascular reactivity was monitored using a real-time data acquisition system (Keithley System 570 Data Acquisition System) and microcomputer (Zenith 241 AT). Blood pressure (BP) was measured from the left upper arm of the subject with a selfcontained computerized oscillometric blood pressure monitor (Dinamap Model 845 XT Vital Signs Monitor, Critikon) connected to the data acquisition system (DAS). The subject's EKG was recorded using a polygraph (Grass Model 7) and Ag/AgCl electrodes

1 This criterion for the habitual caffeine use group was selected because it reflects the average level of daily caffeine use among adults in the United States who are coffee drinkers, estimated by surveys of caffeine consumption to be 4 mg/kg (13). Thus, it was planned that experimental results regarding the development of tolerance could be applied to the large population of average adult caffeine users. Similar selection criteria have also been used in earlier studies of caffeine and stress to represent typical caffeine consumers.

Psychosomatic Medicine 52:320-336 (1990)

CAFFEINE AND STRESS REACTIVITY placed bilaterally on the chest. The EKG was processed by electronic level detection for identification of EKG R-waves. The DAS measured the time intervals between sequential R-waves (heart beat intervals, HBI) with 1-msec precision. Sequences of HBIs were recorded for 45 sec concurrent with BP measurement, and the average HR (bpm) was calculated. BP and HR were measured as described at approximately 1-minute intervals during the experimental conditions. Cardiovascular data were stored by the microcomputer for subsequent reduction and analysis. Respiration was monitored continuously during the study using a pneumatic respiration transducer (Grass) placed around the abdomen below the ribcage to record respiratory excursions. This signal was amplified and recorded by the polygraph. Respiratory excursions were monitored during the study but were not quantified for further analysis.

Blood Sampling and Biochemical Assays Blood samples for analysis of neuroendocrine measures and plasma caffeine level were collected using a continuous exfusion system (Cormed) that provided integrated samples of neuroendocrine activity during 5-min sample periods throughout the study A 19-gauge butterfly needle was inserted into a superficial vein of the subject's right forearm, and specially prepared heparinized tubing extended through the wall of the subject room into the adjoining experimental control room. During collection, blood was withdrawn by the peristaltic exfusion pump at a rate of 1 cc/min into chilled tubes containing glutathione that were kept on ice until the conclusion of the experiment. Blood samples were then centrifuged and the plasma was frozen at —70°C. until biochemical assays were performed. Plasma epinephrine and norepinephrine levels were determined in each sample by assay using HPLC methods with electrochemical detection (15). Plasma cortisol levels were determined by radioimmunoassay using commercial kits. Plasma caffeine levels were determined by radioimmunoassay using a variation of the method developed by Cook et al. (16) with antiserum provided by him.

Experimental Protocol The experiment was completed in three separate sessions. During all experimental procedures, the


subject sat in a comfortable reclining chair, in a semi-recumbent position with feet elevated, inside a 2 x 3-meter room. Cardiovascular monitoring and blood withdrawal equipment were located in an adjacent control room. Orientation session. As in earlier studies (5, 6), the initial laboratory visit was an orientation session designed to acquaint the subject with the procedures of the experiment and to reduce the influence of novelty effects on stress reactivity during later experimental sessions. After completion of informed consent and other enrollment materials, the experiment was explained in detail and methods for cardiovascular monitoring and blood sampling were described and demonstrated. Procedures for venipuncture were described in detail but not demonstrated. As further demonstration, a series of five cardiovascular measurements was conducted while the subject rested quietly for 5 min. The stressful behavioral task (described below) was explained and cardiovascular measurements were recorded while the subject practiced the task for one 5-min trial. The subject's questions were answered and any concerns were addressed before the end of this session in order to insure that the subject understood and was comfortable with the procedures of the next two experimental sessions. Experimental sessions. The caffeine and placebo control treatments were administered in two identical experimental sessions scheduled with 1 to 7 intervening days. Subjects were instructed to maintain consistent routines prior to each study day, to abstain from caffeine following the evening meal prior to each session, and to eat their otherwise normal breakfast on the day of the study.2 At approximately 8:30 A.M.. the subject arrived at the laboratory, height and weight were measured, and the venipuncture was performed in the outpatient pavilion of the DUMC Clinical Research Unit. The

2 This procedure differs from most studies of drug effects that are conducted with subjects in a fasted state. The lack of control over this meal could potentially influence drug absorption or produce variable post-prandial effects on autonomic regulation. Given the delay between breakfast and the administration of caffeine or placebo (estimated to be at least 2 hours), such confounding effects should be minimal, but they must be considered in the interpretation of the results of this study.

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J. D. LANE ET AL. subject returned to the laboratory and relaxed in the subject room during a 1-hour post-venipuncture recovery period. Near the end of this waiting period, equipment for cardiovascular measurements was attached and adjusted, and proper operation of the blood withdrawal system was confirmed. Data collection began with a 15-min resting baseline. The subject was instructed to sit quietly and relax. Ten measurements of blood pressure and heart rate were recorded concurrently with the collection of two 5-min continuous samples of venous blood. Following this initial pre-drug baseline, the appropriate caffeine or placebo treatment was administered. The order of treatments was counterbalanced separately for the two caffeine-use groups and administration was double-blind. Caffeine was administered at a dose of 3.5 mg/kg body weight from a standard solution of anhydrous caffeine in distilled water prepared by the DUMC pharmacy. The prepared caffeine solution also contained a non-pharmacological amount of quinidine (approximately 5 mg per administration) to mask the bitterness of the caffeine The placebo control treatment was a similar solution containing only quinidine. Both standard solutions were diluted with water prior to administration to a total volume of 60 ml (2 oz)-. As in earlier studies using this method, subjects could not discriminate between drug and control treatments. Subjects then rested quietly for 45 min, an interval that the literature suggests (10) would be adequate time for maximal absorption of caffeine after an oral close. After the delay for caffeine absorption, cardiovascular measurements and blood samples were collected during a 15-min post-drug resting baseline while the subject again sat quietly and relaxed. These post-drug resting measurements provided an assessment of caffeine's effects on resting levels of cardiovascular and neuroendocrine activity as well as a baseline for assessment of acute stress reactivity. Following the post-drug resting baseline, subjects were exposed to the laboratory stressor, performance of a challenging serial addition task involving 3-digit numbers. This same stressful task was used in earlier studies of caffeine and cardiovascular reactivity (5, 6). An initial 3-digit number (e.g., 126) was announced to the subject at the beginning of each 5-min task trial. The subject summed the three digits of this number silently, added that sum to the number itself, and reported the resulting answer aloud. These arithmetic routines were repeated with this answer and with each succeeding answer for the duration of the trial. Each of three 5-min trials began with a different starting number and trials

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were separated by a 1-min rest period. Instructions to the subject emphasized both speed and accuracy in the performance of the additions and task performance was tape-recorded. Five cardiovascular measurements and a 5-min integrated blood sample were collected during each task trial within the 20min stress condition. Following the stressor, the subject was instructed to relax and sit quietly once again. Fifteen cardiovascular measurements and three integrated blood samples were collected during the next 20 min to provide data on recovery from the effects of the stressor. This recovery period concluded the experimental session.

Data Reduction and Analysis Each subject received both experimental treatments (caffeine and placebo control) and each session included four experimental conditions (predrug resting baseline, post-drug resting baseline, stress, recovery). Collected data were averaged for each part of the study. The pre-drug and post-drug resting baselines each included data from 10 cardiovascular measurements and two blood samples. The stress and recovery periods each included 15 cardiovascular measurements and three blood samples. One subject did not have neuroendocrine data due to an unsuccessful venipuncture, which reduced the df for analyses of neuroendocrine and plasma caffeine data. The counter-balanced crossover design was analyzed using a 2-Treatment x 4-Condition repeatedmeasures analysis of variance (ANOVA) with profile contrasts (using the GLM procedure, PC-SAS). Treatment order yielded no significant effects or interactions with other factors and was dropped from the analyses. The use of profile contrasts in the repeatedmeasures factor for experimental condition provided, in addition to tests of condition effects and interactions, separate follow-up tests of the effect of Treatment (caffeine vs. control) on the changes (contrasts) from pre-drug to post-drug resting baseline, from post-drug resting baseline to stress, and from stress to recovery. The effects of treatment on the contrast of pre-drug and post-drug resting baselines (designated the Resting Contrast) were examined to evaluate effects of caffeine on resting levels of cardiovascular and neuroendocrine activity. Effects of treatment on the contrast of post-drug and stress values (Stress Contrast) were examined to evaluate drug effects on the responses to the stressor. Greenhouse-Geisser adjustments for significance were


CAFFEINE AND STRESS REACTIVITY used in tests of the repeated measures factors when appropriate. Caffeine-consumption group (Habitual vs. Light) was included as a between-groups factor in analyses directed at the examination of tolerance effects. Additional analyses used calculated scores for the effect of caffeine vs. placebo control that were equivalent to the Resting and Stress contrasts in the ANOVAs. For each dependent variable, the effect of caffeine at rest was calculated by subtracting the change from pre-drug to post-drug resting baseline in the placebo control treatment from the corresponding change in the caffeine treatment. A similar score for the effect of caffeine on reactivity was calculated from the changes from post-drug baseline to the stressful task condition in the caffeine and placebo treatments. These caffeine effect difference scores were used for exploratory data analysis and clarification of ANOVA results.

RESULTS

Effects of Caffeine on Cardiovascular and Neuroendocrine Activity Data from the two groups were combined to evaluate the effects of caffeine treatment on resting activity and stress responses. These combined data are summarized by experimental Treatment and Condition in Table 1. Separate ANOVAs were conducted for each cardiovascular and neuroendocrine variable. Attention was focused on the interactions of experimental Treatment (caffeine vs. control) with experimental Condition and with the Resting and Stress Contrasts (described above). Statistical results for the analyses of these interactions and contrasts are summarized in Table 2. Plasma caffeine JeveJs. Plasma caffeine levels were determined for all four conditions in the caffeine treatment, but only for the pre-drug baseline in the placebo control treatment. Measured levels in the pre-drug baselines were consistent with overnight abstinence, averaging 0.24 ng/ Psychosomatic Medicine 52:320-336 (1990)

ml for the control treatment and 0.37 fig/ ml for the caffeine treatment. The oral administration of 3.5 mg/kg of caffeine increased plasma levels to approximately 4.2 Mg/ml and plasma levels were stable across the three Post-Drug conditions. This finding confirms the adequacy of the 45-min absorption delay and the consistency of the plasma caffeine level across the resting baseline, stress, and recovery conditions.3 Cardiovascular measures. ANOVAs revealed significant main effects of Treatment in SBP and DBP, but not HR. The Treatment by Condition interactions were also highly significant (p < 0.0001) for SBP and DBP, although HR data yielded only a trend (p < 0.11). As seen in Table 1, there was a marked increase in BP from the pre-drug to post-drug resting baseline of 8/8 mmHg (SBP/DBP) after the 3.5 mg/kg caffeine dose, compared to no change in control. These effects on resting BP were confirmed by the highly significant contrasts between pre-drug and post-drug baselines for SBP and DBP (see Resting Contrast, Table 2) Caffeine had a significant effect on the stress reactivity contrast as well, although the increase from the elevated post-drug baseline in the caffeine condition was significantly smaller (7/3 mmHg) than the

3 Subjects varied widely in the plasma level of caffeine resulting from the 3 5 mg/kg dose (range 1.0-10.9 Mg/ml)- The increase in plasma caffeine level after administration was found to be positively correlated with the subject's weight (r = 0.43, p < 0.04), suggesting that the dose/kg administration did not achieve equal plasma concentrations as expected. Higher plasma levels in heavier subjects may have resulted from increased percentages of fat to lean body mass and the fact that caffeine distributes into tissue according to water content.

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J. D. LANE ET A L TABLE 1. Effects of Caffeine (3.5 mg/kg) on Cardiovascular and Neuroendocrine Responses at Rest and During a Challenging Mental Arithmetic Task" Experimental Condition Variable (units) SBP (mmHg) DBP (mmHg) HR (bpm) Epi (pg/ml) Norepi (pg/ml) Cortisol (ng/ml) Caffeine (Mg/ml)

Drug Treatment Caffeine Control Caffeine Control Caffeine Control Caffeine Control Caffeine Control Caffeine Control Caffeine Control Control

Pre-Drug Rest

Post-Drug Rest

Stress

Recovery

112 + 2 112 dt 2 63 ± 2 64 ± 2 62 ± 2 2 64 ± 2 21.6 ±2.5 27.0 ±4.3 227± 17 222 ± 12 58.5 ± 6.3 59.2 ±6.9 0.37 ±.13 0.24 .04 0.24 ± ± .04

120 ± 2 " 112 ± 2 71 ± 2 " 64 ± 2 62 ± 2 62 ± 2 36.9 ±4.8 29.5 ±5.1 330 ± 3 6 2 256+18 59.5 ± 6.7 56.2 ± 7.8 4.06 ± .51

127 ± 2 3 121 ± 2 74 ± 2 2 70 ± 2 72 ± 2 73 ± 2 56.8 ± 6 . 9 ' 38.6 ±4.6 329 ± 28' 279 ± 2 2 78.9 + 9 . 1 2 65.1 ±7.3 4.29 ±.56

120 ± 2 2 116 ± 2 67 ± 2 2 64 ± 2 63 ± 2 63 ± 2 59.8 ± 10.7' 30.7 ±5.4 306 ± 24 288 ± 24 89.1 ± 10.3' 73.9 ±6.8 4.20 ± .46

Data are means ± SEM. Superscripted numbers associated with means in the caffeine condition reflect ignificance of differences between caffeine and placebo control treatments during that experimental condition valuated by paired-! test, p < 0.05,2 p < 0.01, J p < 0.001," p< 0.0001

TABLE 2. Effects of Drug Treatment (Caffeine vs. Control), Interactions with Experimental Conditions, and Drug Effects on Repeated-measures Contrasts

SBP DBP HR Ep, Norepi Cortisol

Drug Treatment

Drug x Condition Interaction

Resting Contrast

Stress Contrast

Recovery Contrast

F(1,23)J

F(3,69)

F(l,24)

F(1,24)

F(l,24)

18.3O3 19.263 0.65 4.51' 3.07 7.522

20.59" 23.41 4 2.20 6.982 3.59' 2.64

53.06" 72.78" 3.80 4.78' 4.11' 0.31

6.21' 14.803 0.12 4.27' 1.49 5.591

4.15' 1.39 0.43 2.12 4.34' 0.06

" Degrees of freedom for cardiovascular measures Neuroendocrine variables have slightly reduced df due to loss of one subject's data. Superscripted numbers reflect significance of test after Creenhouse-Ceisser adjustment when appropriate. 1 p < 0.05, 2 p < 0.01, 3 p < 0.001, 4 p < 0.0001

increase in the control condition (9/6 mmHg). Despite the smaller stress response, BP was significantly higher (by 38 mmHg} when caffeine and control treatments were compared in all three experimental conditions that followed caffeine 326

administration (see Table 1). Caffeine added to the effects of stress to elevate BP, even though it was associated with an apparent attenuation of the stress response measured from the elevated postdrug resting baseline. Psychosomatic Medicine 52:320-336 (1990)


Neuroendocrine measures. The main effect of experimental Treatment was significant for both plasma cortisol and epinephrine, although norepinephrine data yielded only a trend approaching significance (p < 0.09), and the caffeine treatment was associated with higher plasma levels of each hormone (Table 1). The more important Treatment by Condition interaction was significant for both epinephrine and norepinephrine, although cortisol data revealed only a trend nearing significance (p < 0.07) after adjustment for repeated measures (Table 2). The interactions of caffeine treatment and experimental condition for the neuroendocrine variables are presented in Figure 1. Investigation of the interactions and contrasts (Table 2) revealed that caffeine significantly increased resting plasma levels of epinephrine and norepinephrine (from pre-drug to post-drug baseline) compared to control, but did not affect resting plasma cortisol levels. The figure and associated tests of the contrasts also demonstrate clearly that caffeine also markedly potentiated the epinephrine and cortisol stress responses, more than doubling the responses in the control condition. The epinephrine stress response after caffeine treatment was 233% of the stress response in the control condition. The cortisol stress response after caffeine was 211% of the control stress response. Comparisons of the effects of caffeine treatment during each experimental condition revealed that caffeine elevated resting levels of norepinephrine and epinephrine, but not resting levels of cortisol (Table 2). In addition, caffeine treatment was associated with significantly elevated plasma levels of epinephrine, norepinephrine, and cortisol during the experimental stressor and higher epinephrine and cortisol levels in the post-stress recovery Psychosomatic Medicine 52:320-336 (1990)

350r Norepinephrine

Q.

200

70r

D) Q.

Epinephrine

O

10 100 r

Cortisol • Caffeine O Control

Q.

50

PB DB ST RC

Fig. 1. Effects of caffeine vs. placebo control treatment on measures of neuroendocrine stress reactivity during the Pre-Drug Baseline (PB), Post-Drug Baseline (DB), Psychological Stress (ST), and Post-Stress Recovery (RC).

period. Figure 1 also suggests that caffeine treatment was associated with a persistence of the epinephrine and cortisol stress responses during the 20-min recovery 327

J. D. LANE ET AL.

period. Epinephrine levels remained elevated, at the levels observed during performance of the stressful task, rather than returning to resting levels as in the control treatment. Cortisol levels continued to rise during recovery in both the caffeine and control treatments, but caffeine treatment was associated with significantly higher values suggesting a persistent enhancement of stress effects.

Effects of Habitual Caffeine Use Descriptive data for the habitual user and light user groups are presented in Table 3. Although no attempts were made to match subjects in these two caffeineuse groups, the groups did not differ in age, height, weight, or in resting levels of cardiovascular and neuroendocrine activ-

ity. The groups differed only in the level of daily caffeine consumption, based on both the initial self-report and the caffeine diary. The non-user group averaged daily caffeine consumption near 60 mg, below the level found in a 5-oz cup of coffee and consistent with recruitment criteria. Values based on self-reports ranged from 0135 mg/day and diary estimates [n = 11) ranged from 0-163 mg/day. The habitual caffeine-user group averaged over 500 mg/day of caffeine, higher than the planned range of 250-350 mg/day that represents average adult use. Their selfreports ranged from 285-960 mg/day and diary estimates (n = 10) from 204-1690 mg/day. Despite the clear differences in daily caffeine consumption, the habitual caffeine-user and non-user groups did not differ in either the plasma caffeine level

TABLE 3. Comparisons of Habitual Caffeine-Users and Light Users" Croup Age (yrs) Height (cm) Weight (kg) Reported Caffeine Consumption (mg/day) Self-report 1-Week diary Resting Physiology'' SBP (mmHg) DBP (mmHg) MAP (mmHg) HR(bpm) EPI (pg/ml) NOREPI (pg/ml) CORT (ng/ml) Plasma Caffeine Levels'" Pre-Drug Post-Drug

Habitual Users (n = 13)

24.9 ± 1.7 180.1 ± 2 . 3 75.7 ± 3 . 0

29.3 ± 1.6 179.5 ± 2 . 0 76.1 ±2.3

514.2 ±64.3 567.7 ± 147.1 111.5 63.3 78.5 62.6 22.4 222.2 53.6

± 2.4 ±2.7 ±22 ±3.3 ± 3.4 ± 17.0 ±6.3

0.39 ±0.15 4.31 ± 0.81

Light Users (n=12)

60.7 ± 16.5 53.9 ± 16.5 113.0 64.1 80.1 62.1 26.2 226.8 64.1

± 2.4 ± 1.9 ± 2.0 ±2.2 ± 5.3 ± 22.4 ± 10.3

0.22 ±0.05 4.05 ± 0.62

t(p)

1.90 (0.07) -0.18 (ns) 0.13 (ns)

6.80(0.0001) 3.50 (0.0082) -0.44 -0.24 -0.55 0.13 -0.60 -0.16 -0.85

(ns) (ns) (ns) (ns) (ns) (ns) (ns)

1.06 (ns) 0.24 (ns)

•' Data are means ± SEM. Comparisons of habitual users and non-users by (-test, df = 23. (ns = p > 0.30). 6 Average of the pre-drug resting baselines in both experimental sessions. r Pre-drug values averaged for both caffeine and placebo treatments. Post-drug levels averaged across postdrug resting baseline, stress, and recovery conditions, caffeine treatment only.

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prior to experimental administration or in the levels reached at any point in the study after administration of caffeine (t test, all p > 0.20). Several approaches were used to investigate whether habitual caffeine use was associated with reduced caffeine effects attributable to the development of tolerance. The planned analysis included Caffeine-Use Group (habitual vs. light) as a between-subjects variable in ANOVAs that included the within-subject factors of experimental Treatment and Condition. These univariate ANOVAs, conducted for each of the cardiovascular and neuroendocrine variables, yielded no significant Group x Treatment or Group x Treatment X Condition interactions, and only one non-significant trend was observed (for cortisol, Group X Treatment, p < 0.07). The results are summarized in Table 4. After the failure of the ANOVAs to detect group differences modulating caffeine effects, linear regression analyses were conducted with the level of reported daily caffeine consumption included as a continuous independent variable in the GLM. Linear regression would be ex-

pected to provide a more powerful test for tolerance effects, given the observed variability in reported levels of caffeine consumption among the individuals within both Caffeine-Use Groups. Both the questionnaire reports (n = 25) and the caffeine diaries (n = 21) were examined in regression analyses that included Treatment and Condition factors. These analyses also failed to yield significant main effects for the reported level of caffeine consumption or interactions of intake level with Treatment or Treatment and Condition in any of the cardiovascular or neuroendocrine measures. None of the appropriate tests yielded a p < 0.15. Further analyses were conducted using the calculated difference scores for caffeine effects on resting level and response (see above for derivation). Although these additional analyses were redundant with the ANOVAs and increased the possibility of false positive findings, these difference scores were examined to provide the most direct and powerful tests for the presence of caffeine tolerance in the data. The results were still negative. Comparisons of habitual and light consumer groups by t test (df = 22 or 23) revealed no significant

TABLE 4. Summary of ANOVAs for Comparisons of Habitual Caffeine Users and Non-Users (Group) and Interactions with Drug Treatment and Experimental Condition Croup F(1,23) SBP DBP HR

Epi Norepi Cortisol

Croup x Drug F(1,23)

Croup x Drug x Condition F(3,69)

0.33 0.00 0.05

0.10 0.08 0.07

0.55 0.63 0.52

F(l,22)

F(1,22)

F(3,66)

0.16 0 22 0.90

0.81 0.32 3.70J

2.15 1.33 0.42

'p 0.10), even gest the presence of "ceiling effects" for when the regression equations also con- the increases associated with the combitrolled for body mass index (kg/m2) and nation of caffeine and stress, but given actual dose of drug administered. The var- that these correlations are based on obiations in the increase in the plasma level servations of independent subjects, they of caffeine apparently did not modulate may also reflect individual differences in the observed effects of caffeine on cardi- the overall response to the drug. 330



BP stress responses measured from this elevated baseline were smaller than reThe present study had two objectives, sponses in the control condition and the to assess the effects of caffeine on neu- magnitude of caffeine's effect on the presroendocrine stress reactivity and to ex- sor stress responses was negatively coramine whether the habitual consumption related with the elevation produced at of caffeine was associated with the devel- rest. These observations suggest that a opment of tolerance, represented by a "ceiling effect" may have been observed diminution or elimination of caffeine's ef- for the combination of caffeine and stress. fects. The results presented above provide Despite this, BP was significantly higher answers to both questions. They show during the performance of the stressful that the effects of caffeine to supplement task when caffeine had been adminisor potentiate stress reactivity are not lim- tered, a result which suggests that cafited to cardiovascular responses, but in- feine may raise BP during more natural clude measures of neuroendocrine stress stressors as well. The absence of clear reactivity as well. The administration of effects for HR is also consistent with eara moderate dose of caffeine produces el- lier research studies and may reflect the evations in plasma norepinephrine that presence of mixed vagal and sympathetic are additive to the effects of stress and caffeine actions. The positive correlations also produces a clear potentiation of epi- among the caffeine effects on SBP, DBP, nephrine and cortisol stress reactivity. and HR stress responses suggest that cafThe comparisons of habitual moderate feine might be affecting an integrated patcaffeine consumers and light consumers tern of cardiovascular activation, perhaps found no evidence that the level of daily that associated with the expression of adcaffeine consumption modulates these ob- renergically mediated sympathetic nervserved effects of caffeine. The absence of ous system activation to stressful stimuli. significant effects argues that habitual Our neuroendocrine findings provide a caffeine use does not necessarily lead to complement to the earlier research on tolerance and suggests that the observed caffeine potentiation of cardiovascular reinteractions of caffeine and stress may be activity and demonstrate a consistency present even in individuals who habitu- between these two aspects of the "fightally consume moderate amounts of caf- flight" response. Other investigators (9) feine. have reported similar effects of caffeine on norepinephrine and cortisol, including the additive effects of caffeine and stress on plasma norepinephrine and the potenEffects of Caffeine on Cardiovascular tiation of cortisol stress reactivity that are and Neuroendocrine Activity confirmed in the present study. However, The cardiovascular effects of caffeine the earlier study did not measure epiare consistent with earlier studies and nephrine stress reactivity, which we find have the same interpretation. Caffeine was magnified significantly (more than raised resting levels of BP and apparently 200%) by caffeine. This observation may added to the level of BP reached during be the clearest evidence to date of a postress by raising the baseline from which tentiation of the "fight-flight" response in the stress-related pressor response begins. the neuroendocrine system resulting from DISCUSSION


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the consumption of a moderate dose of caffeine. The pattern of effects present in cardiovascular and neuroendocrine responses suggests that changes in sympathetic nervous system neurotransmitter and neurohumoral activity, as reflected in changing plasma levels of stress hormones, could underlie the cardiovascular effects seen in this and in earlier studies. The increase in resting plasma levels of norepinephrine is consistent with the rise in resting BP that is mediated by increases in peripheral resistance rather than cardiac output (17). Both may reflect increases in sympathetic nervous system activity mediated by norepinephrine stimulation of a-adrenergic receptors. The potentiation of epinephrine stress reactivity is consistent with the increases in skeletal-muscle blood flow and vasodilation (5, 6), which are mediated by /32-adrenergic receptors stimulated by circulating epinephrine. The epinephrine response could also mediate the enhancement of the cardiac output response and the decrease in total peripheral resistance seen by other investigators during performance of a stressful task (9). This model of an integrated pattern of caffeine effects on stress reactivity is weakened by the absence of correlations between caffeine's cardiovascular and neuroendocrine effects, with the exception of effects on resting norepinephrine and heart rate. However, the cardiovascular variables that are most closely related to the neuroendocrine effects (i.e., forearm blood flow and cardiac output or total peripheral resistance) were not measured in the present study. Further examination of these variables in future studies is certainly indicated. The lack of correlational evidence for an integrated cardiovascular and neuroendocrine pat332

tern of response could also be the result of the lack of heterogeneity among subjects in the effects of the caffeine administration, which might have led to a restriction of the range of values. The Effects of Habitual Caffeine Consumption The present study finds no evidence that the level of habitual caffeine consumption affects the magnitude of the observed caffeine effects. Although earlier studies of habitual caffeine consumers (68) reported significant effects of caffeine on cardiovascular stress reactivity similar in magnitude to effects reported in nonconsumers (4, 5), the present study is the first to examine directly the effects of different levels of habitual caffeine consumption with respect to effects on acute stress reactivity. The appearance of significant caffeine effects in both caffeine consumption groups and the failure to find even non-significant trends linking habitual consumption to reduced caffeine effects provide strong evidence that the observed caffeine/stress interactions can persist in habitual caffeine consumers. Habitual consumption of caffeine does not necessarily lead to the development of tolerance. Negative results must always be interpreted cautiously. Several issues could be raised to question our interpretation of the negative results in the present study. The apparent absence of caffeine tolerance could be due only to the limitations of the present study to detect such effects, resulting from inadequate statistical power to detect reasonable evidence of tolerance or because the two groups did not differ meaningfully in their consumption levels. These possibilities are unlikely in the present study. Psychosomatic Medicine 52:320-336 (1990)


The experiment was designed to provide acceptable statistical power [80% likelihood of success) to detect group differences in caffeine effects greater than 1.2 times the within-group standard deviation by t test. The sample also provided reasonable power in correlational analyses (75% likelihood of success) to detect r values of 0.50 or larger, relationships for which level of habitual caffeine intake would account for at least 25% of variability in caffeine's effects. These lower limits of detection were judged to be acceptable after a consideration of what would constitute meaningful evidence of tolerance, based on a review of the data collected in our earlier studies of caffeine and stress. A stronger argument that the absence of tolerance effects is not due to limitations in statistical power emerges from an examination of the actual sizes for differences between habitual and light users, calculated retrospectively from the data of the present study. These effect sizes were calculated for the two-group comparisons (t test) on the resting and response caffeine effect scores and for the comparable correlational analyses described above and yielded two important observations. First, the direction of nonsignificant differences suggested larger caffeine effects in the habitual-use group in five of the 14 t-tests and half of the correlations, which contradicts the tolerance hypothesis. Second, the calculated effect sizes for the t-tests (d = m, — m2/